The addition of fillers to polymers is a common industrial practice. Its purpose is generally twofold; to reduce the overall cost of the composite while concurrently bringing about improvement in, for example, mechanical properties such as wear, hardness, tensile modulus, tear etc. It is also known that by using filler particles of very small dimensions (<200 nm), polymer composite properties may be improved with a much lower concentration of filler, typically 2 to 10 parts by weight per 100 parts by weight of rubber compared to 30 to 100 parts by weight per 100 parts by weight of rubber with normally micron sized filler particles. Nanometer or “nano-scale” fillers possess extremely high surface area with high surface energy. For this reason, it is important that this surface energy be overcome or dissipated in some manner to allow efficient compatibilization and dispersion of the nano-filler into the polymeric substrate to avoid filler aggregation during processing and finishing of the final product.
Formation of organic-inorganic nanocomposites based on clays or layered silicates such as montmorillonite is known. Generally, these clays are normally modified by alkyl ammonium ions or amines through an ion-exchange process. The large organic ions displace the smaller inorganic ions (e.g. sodium ion) that reside between the negatively charged silicate platelets thus expanding the interplate distances while concurrently making the modified clay more hydrophobic. The end result is a modified clay that is more readily dispersible in a polymeric substrate. In some instances, the new nanocomposite has an intercalated structure wherein the clay is well dispersed with no large aggregates but still largely retains its layered (but expanded) domain morphology. The expanded layered structure will allow some of the polymeric substrate to interpenetrate between the stacked platelets which, in turn, often results in physical property improvements. While the intercalated polymer-clay domain structure is an improvement, it does not reach the ultimate state of dispersion commonly referred to as exfoliation. When an exfoliated state is achieved as evidenced by a complete lack of an X-ray diffraction pattern, the layered structure of the clay is completely disrupted. Individual platelets in this state now have little affinity for each other relative to the polymeric substrate. The attainment of the exfoliated state will provide the most improvement in properties with the lowest level of filler possible.